Methods of continuously or in-line forming a seamed metal tube from a continuous strip or skelp are well known. After cleaning and edge conditioning of the strip, if required, the skelp is rolled to form an open seam tube having nearly abutting edges at the top of the tube. The edges are then welded together by one of several methods which generally include heating the edges and either forging the edges together with squeeze rolls and/or continuous flux welding. The edges of the tube may be heated for example by resistance welding, an electric arc or by high frequency induction welding. High frequency welding is a form of electric resistance welding, wherein the open seam tube is received through an electric coil which creates a strong magnetic field, which in turn induces current to flow around the tube. An impeder is generally located within the tube which forces the current flow down the nearly abutting edges of the open seam tube, heating the tube edges to a hot forging temperature. The tube edges are then forged by squeeze rolls which drive the molten edges together to form an integral seam.
In-line galvanizing and coating or painting processes are also well known. The strip or skelp may be galvanized or painted on one or both surfaces prior to forming and welding or the welded seamed tube may be galvanized by immersion in a molten zinc bath.
Where the strip is coated with a protective coating prior to seam welding, the coating will burn off or melt in the seam zone because the welding operation involves the melting of the tube material, which is generally steel. Thus, the welding temperature may be 2,300.degree. F. or greater. Where the strip has been painted, the paint will burn off in the weld zone. Where the strip is coated with a metal, such as zinc or aluminum, the metal will melt and flow downwardly away from the seam, which is located at the top of the tube. A zinc coating solution has also been used to paint the exterior surface of the seam. However, such a coating is primarily cosmetic and has poor adhesion. The failure of present processes to fully coat and thus protect the tube seam is evident by the fact that the weld area is generally the first to fail in accelerated corrosion tests. Thus, there has been a long-felt need to provide an improved coating process, particularly on the seam.
Further, the art has long recognized the advantages of galvanizing ferrous tubing in an inert atmosphere, such as nitrogen. Maintaining a reducing or nonoxidizing atmosphere within the galvanizing tank improves the resultant zinc coating, reduces oxidization or rust and waste, including slag. However, the prior art has failed to develop a commercially acceptable galvanizing apparatus which can be serviced and which maintains an inert or nonoxidizing atmosphere, particularly during service.
The continuous tube forming and coating process and apparatus of this invention solves the above-identified problems and produces a superior coated tube. The process of this invention assures a fully coated inner seam and an improved galvanized exterior tube surface.